Replace Cron Expression with RRule #2
Description
Right now polly uses a cron expression that is evaluated but not really flexible when it comes to stuff like 'last monday of month' or so.
Using RRules as defined by iCalendar (https://www.rfc-editor.org/rfc/rfc5545) seems to be a viable replacement.
https://icalendar.org/iCalendar-RFC-5545/3-8-5-3-recurrence-rule.html